1. Field of the Invention
The present invention relates to a refrigerator having a refrigerant circulating circuit in which a mixture of a refrigerant and a refrigerating machine oil is circulated.
2. Description of the Prior Art
A typical conventional refrigerator air for vehicles is shown in FIG. 6. A refrigerant such as freon gas is circulated in refrigerant circulating circuit 1 formed from pipe 2. Compressor 3, condenser 4, receiver dryer 5, expansion valve 6 and evaporator 7 are provided in circulating circuit 1 sequentially in the direction of circulation of the refrigerant, shown by arrows. The endothermic surface of evaporator 7 is exposed to the interior of the vehicle (not shown). After the refrigerant is compressed by compressor 3, the refrigerant is transformed in phase from a high-pressure gas to a high-pressure liquid in condenser 4 and further to a low-pressure gas as it passes through expansion valve 6 and evaporator 7. When the refrigerant is transformed from a liquid phase to a gaseous phase (vapor phase) by evaporator 7, the refrigerant absorbs heat from the interior of the vehicle and the vehicle interior is cooled. In such a refrigerator, a mixture of a refrigerant and a refrigerating machine oil is usually used as the refrigerant circulated in the circulating circuit 1. The refrigerating machine oil lubricates the slidable portions of compressor 3.
Although freon has been mainly used as the refrigerant for such a refrigerator, recently substitute refrigerants for freon have been used. Among refrigerating machine oils used for such substitute refrigerants, there are some oils which cannot sufficiently dissolve in the refrigerants. In such a case, the compatibility between the refrigerant and the refrigerating machine oil depends on temperature. Namely, if the temperature of a mixture of a refrigerant and a refrigerating machine oil is high, the refrigerant and the refrigerating machine oil are separated to a two phase mixture. As the result of such a two phase separation, a part of the refrigerating machine oil stagnates in some portions in circulating circuit 1, and an insufficient amount of the refrigerating machine oil is returned to compressor 3. Consequently, there is a possibility that compressor 3 may lock up or break.